Stibnite

from South Africa --- antimony sulphide, Sb2S3

Here are two photomicrographs in offset-crossed-polarized
reflected light of stibnite crystals in gold-antimony
(Au-Sb) ore from an underground mine in the Murchison greenstone belt on the
Archean Kaapvaal craton, south of the Limpopo river in
the northeast corner of South Africa.

Stibnite is the major ore mineral of antimony (Sb).
Antimony is a metal used to harden lead alloys for use in batteries and other applications.
It is an ingredient of other speciality alloys, such as pewter,
Babbitt metal and type-metal. Antimony oxides are used
in paint pigments, as colouring agents in glass, and as fire retardents.

This gold-antimony ore contains an estimated 15 volume percent of the
sulphide, much less than the hand-
specimen appearance suggests, due to a tendency for the samples to
break along stibnite-lined fracture surfaces. Minor pyrite and
arsenopyrite also occur in the quartz-carbonate matrix.

Stibnite is a soft, orthorhombic mineral,
steel-grey with a
bright metallic lustre, specific gravity 4.63.
In coarse-grained occurrences it is notable for forming
spectacular, highly elongate, striated prisms, popular with
mineral collectors.
It is greyish-white in reflected light, with strong bireflectance
and anisotropy and a marked tendency to undulose extinction.
China has roughly 50% of
world reserves of Sb and accounts for some 80% of world Sb production.
The main producers (85% of the industry) are in Hunan province, but at least 12
regions of China have Sb deposits (Wu, 1993; Behling et al.,
2002).

STIBNITE FROM THE MURCHISON BELT, SOUTH AFRICA

Local Au-Sb deposits are hosted in talc-carbonate and quartz-
carbonate schists, formed at least in part by carbonatization of
komatiites (Pearton, 1979; Pearton and Viljoen, 1986). Au-Sb
mineralization is epigenetic and syntectonic, largely as quartz-
carbonate- stibnite veins in syn-D1 brittle fractures in massive
quartz-carbonate rocks (Boocock et al., 1988) related to shear
zones, many of which host brittle-ductile vein systems (Nutt et al.,
1988). Ore on the local "Antimony Line" is confined to a 10-25-m zone of
massive quartz-carbonate rock in an envelope of schistose chloritic
quartz-carbonate rock and talc-carbonate schist (Willson and
Viljoen, 1986).

The stibnite-rich sample was collected in November
1995 during a field trip of the Mineral Deposits Division of the
Geological Association of Canada and the Society of Economic Geologists,
organized by Dani Alldrick,
with the assistance of geologist Colin Willson of the Consolidated
Murchison Mine.

The mineralogical residence of gold in the ore
is not apparent in this particular sample.
No native gold was seen in the
three polished mounts of the ore.
If these mounts are representative, much of
the gold may be in solid solution or submicroscopic inclusions in
stibnite and arsenopyrite, consistent with the limited occurrence of
native gold and aurostibite in the belt (Abbot et al., 1986).
A brief energy-dispersive analysis using the proton microprobe facility at the
University of Guelph (Ontario, Canada)
revealed minimal impurities, with a nominal detection limit
of 12 ppm for Au (Turnstone Reports 1996-100 and -101).
Putting a very conservative upper bound of 50 ppm on the gold content
of the stibnite, this sulphide may nevertheless contain
a high proportion of the "invisible gold" in this ore.

Some common ore minerals such as chalcopyrite tend to
be relatively pure, with impurities in most cases <0.1 wt.%,
whereas others show more variability, e.g., pyrite in some
geological environments contains low percent levels of arsenic, and
sphalerite many contain 10 percent or more iron and several tenths
of 1 percent cadmium
substituting for the zinc.